Vanadium Thiolate Complexes for Efficient and Selective Sulfoxidation Catalysis: A Mechanistic Investigation

The structural and electronic properties
as well as the catalytic activity toward sulfoxidation of two new
vanadium complexes have been investigated. They both possess in their
coordination sphere two alkyl thiolate ligands: a dioxido VV complex [VO2LNS2]­(HNEt3) (1) (LNS2 = 2,2′-(pyridine-2,6-diyl)­bis­(1,1′-diphenylethanethiol))
and an oxido VIV complex [VOLN2S2] (2) (LN2S2 = 2,2′-(2,2′-bipyridine-6,6′-diyl)­bis­(1,1′-diphenylethanethiol)).
The X-ray structure of 1 has revealed that the VV metal ion is at the center of a distorted trigonal bipyramid.
The optimized structure of 2 obtained by DFT calculations
displays a square-pyramidal geometry, consistent with its EPR spectrum
characterized by an axial S = 1/2 signal (g⊥ = 1.988, g∥ = 1.966, Ax(V) = 45
× 10–4 cm–1, Ay(V) = 42 × 10–4 cm–1, Az(V) = 135 × 10–4 cm–1). DFT calculations have shown that the HOMO (highest occupied molecular
orbital) of 1 is notably localized on the two thiolate
sulfur atoms (56% and 22%, respectively), consistent with the expected
covalent character of the VV–S bond. On the other
hand, the SOMO (singly occupied molecular orbital) of 2 is exclusively localized at the VIV ion (92%). Complexes 1 and 2 have shown an ability to catalytically
oxidize sulfide into sulfoxide. The oxidation reactions have been
carried out with thioanisole as substrate and hydrogen peroxide as
oxidant. Yields of 80% and 75% have been obtained in 10 and 15 min
for 1 and 2, respectively. However, in terms
of conversion, 1 is more efficient than 2 (81% and 44%, respectively). More importantly, the reaction is completely
selective with no trace of sulfone produced. While 1 displays
a poor stability, catalyst 2 shows the same efficiency
after five successive additions of oxidant and substrate. The difference
in reactivity and stability between both complexes has been rationalized
through a mechanism study performed by means of experimental data
(51V NMR and EPR spectroscopy) combined with theoretical
calculations. It has been shown that the structure of the cis-oxo peroxo VV intermediate species, which
is related to its stability, can partly explain these discrepancies.